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Allyl/Alkyne Coupling Reactions Mediated by Neutral Ruthenium(II). Isolation and Characterization of Ruthenium(II) η3-Allyl η2-Alkyne Complexes Christina M. Older and Jeffrey M. Stryker* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received March 30, 2000 Summary: In contrast to the cyclization products obtained from allyl/alkyne coupling reactions in the isostructural cationic series, neutral complexes of the form (C5Me5)Ru(η3-allyl)(η2-alkyne) react to give acyclic η5pentadienyl complexes in high yield. Investigations of transition-metal-mediated allyl/ alkyne coupling reactions have revealed a variety of synthetically valuable or potentially exploitable reactivity patterns.1-8 In particular, metal-mediated allyl/ alkyne cycloaddition reactions using cationic latetransition-metal complexes of the form [(CnRn)M(η3(1) Catalytic or stoichiometric organic reactions involving coupling of η3-allyl and alkyne ligands (reviews and recent references): (a) Chiusoli, G. P.; Cassar, L. Angew. Chem., Int. Ed. Engl. 1967, 6, 124. Chiusoli, G. P. Acc. Chem. Res. 1973, 6, 422. (b) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 186. (c) Billington, D. C. Chem. Soc. Rev. 1985, 93 and references therein. (d) Oppolzer, W.; Xu, J.-Z.; Stone, C. Helv. Chim. Acta 1991, 74, 465. Oppolzer, W. Pure Appl. Chem. 1990, 62, 1941. (e) Camps, F.; Moreto´, J. M.; Page`s, L. Tetrahedron 1992, 48, 3147. Vilaseca, F.; Page`s, L.; Villar, J. M.; Llebaria, A.; Delgado, A.; Moreto´, J. M. J. Organomet. Chem. 1998, 551, 107. Garcı´aGo´mez, G.; Moreto´, J. M. J. Am. Chem. Soc. 1999, 121, 878. Gomez, G. G.; Camps, X.; Cayuela, A. J. Moreto´, J. M. Inorg. Chim. Acta 1999, 296, 94. (f) Cui, D.-M.; Tsuzuki, T.; Miyake, K.; Ikeda, S.-I.; Sato, Y. Tetrahedron 1998, 54, 1063 and references therein. (g) Montgomery, J.; Savchenko, A. V. J. Am. Chem. Soc. 1996, 118, 2099. Montgomery, J.; Oblinger, E.; Savchenko, A. V. J. Am. Chem. Soc. 1997, 119, 4911. (h) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 1999, 121, 9728. (i) Ferna´ndez-Rivas, C.; Me´ndez, M.; Echanvarren, A. M. J. Am. Chem. Soc. 2000, 122, 1221. (2) Coupling of η1-allyl complexes and alkynes: Welker, M. E. Chem. Rev. 1992, 92, 97. (3) (a) Green, M.; Taylor, S. J. J. Chem. Soc., Dalton Trans. 1975, 1142. (b) Appleton, T. G.; Clark, H. C.; Poller, R. C.; Puddephatt, R. J. J. Organomet. Chem. 1972, 39, C13. (c) Sbrana, G.; Braca, G.; Benedetti, E. J. Chem. Soc., Dalton Trans. 1975, 755. (d) Fischer, R. A.; Herrmann, W. A. J. Organomet. Chem. 1989, 377, 275. (e) Betz, P.; Jolly, P. W.; Kru¨ger, C.; Zakrzewski, U. Organometallics 1991, 10, 3520. (f) Barrado, G.; Hricko, M. M.; Miguel, D.; Riera, V.; Wally, H.; Garcı´a-Granda, S. Organometallics 1998, 17, 820. (g) Biasotto, F.; Etienne, M.; Dahan, F. Organometallics 1995, 14, 1870. (h) Greco, A.; Green, M.; Stone, F. G. A. J. Chem. Soc. A 1971, 3476. (i) Bottrill, M.; Green, M.; O’Brien, E.; Smart, L. E.; Woodward, P. J. Chem. Soc., Dalton Trans. 1980, 292. (j) Davidson, J. L.; Green, M.; Stone, F. G. A.; Welch, A. J. J. Chem. Soc., Dalton Trans. 1976, 2044. (k) Tang, J.; Shinokubo, H.; Oshima, K. Organometallics 1998, 17, 290. (4) (a) Lutsenko, Z. L.; Kisin, A. V.; Kuznetsova, M. G.; Bezrukova, A. A.; Khandkarova, V. S.; Rubezhov, A. Z. Inorg. Chim. Acta 1981, 53, L28. (b) Lutsenko, Z. L.; Kisin, A. V.; Kuznetsova, M. G.; Rubezhov, A. Z. Bull. Acad. Sci. USSR (Engl. Transl.) 1981, 1141. (c) Lutsenko, Z. L.; Rubezhov, A. Z. Bull. Acad. Sci. USSR (Engl. Transl.) 1982, 1259. (d) Lutsenko, Z. L.; Aleksandrov, G. G.; Petrovskii, P. V.; Shubina, E. S.; Andrianov, V. G.; Struchkov, Y. T.; Rubezhov, A. Z. J. Organomet. Chem. 1985, 281, 349. (e) Lutsenko, Z. L.; Petrovskii, P. V.; Bezrukova, A. A.; Rubezhov, A. Z. Bull. Acad. Sci. USSR (Engl. Transl.) 1988, 735. (5) Nehl, H. Chem. Ber. 1993, 126, 1519. (6) Schwiebert, K. E.; Stryker, J. M. Organometallics 1993, 12, 600. (7) Older, C. M.; Stryker, J. M. Organometallics 1998, 17, 5596. (8) (a) Schwiebert, K. E.; Stryker, J. M. J. Am. Chem. Soc. 1995, 117, 8275. (b) Etkin, N.; Dzwiniel, T. L.; Schwiebert, K. E.; Stryker, J. M. J. Am. Chem. Soc. 1998, 120, 9702. (c) Dzwiniel, T. L.; Etkin, N.; Stryker, J. M. J. Am. Chem. Soc. 1999, 121, 10640.
allyl)(η2-alkyne)]+ (eq 1) have been used to prepare
substituted η5-cyclopentadienyl complexes4-7 and, more recently, η5-cycloheptadienyl complexes.8 To probe the effects of reducing the electrophilicity of the metal without substantially altering the molecular orbitals involved, we have investigated the chemistry of the isostructural but neutral (C5Me5)RuII template. In this communication, we report the synthesis of a range of isolable (C5Me5)Ru(η3-allyl)(η2-alkyne) complexes, which undergo allyl/alkyne coupling to yield acyclic η5-pentadienyl complexes rather than cycloadducts. Despite the kinetic lability of most allyl alkyne complexes, (C5Me5)Ru(η3-allyl)(η2-PhCtCPh) (3a) can be synthesized by simply treating a THF solution of (C5Me5)Ru(η3-allyl)X2 (X ) Br, Cl)9 1 with Rieke zinc10 (1012 equiv) in the presence of diphenylacetylene (eq 2).11 Complex 3a was isolated as an air-stable orange material by trituration of the crude residue into pentane. Analytically pure complex 3a was obtained by recrystallization from cold pentane (76%).12 The exo orientation of the allyl ligand in complex 3a was confirmed both by difference NOE spectroscopy and by X-ray crystallography (vide infra). While this complex is quite stable in the solid state, slow conversion to the endo isomer is observed upon standing in benzene solution at room temperature (ca. 30% conversion after 24 h). The corresponding 2-butyne complex (C5Me5)Ru(η3allyl)(η2-MeCtCMe) (3b) can be prepared in a similar manner, although it is thermally more labile than diphenylacetylene complex 3a. Prolonged exposure of 3b at room temperature to either vacuum or the atmosphere results in decomposition to unidentifiable products, yet this complex is indefinitely stable in the (9) (a) Nagashima, H.; Mukai, K.; Shiota, Y.; Yamaguchi, K.; Ara, K.; Fukahori, T.; Suzuki, H.; Akita, M.; Moro-oka, Y.; Itoh, K. Organometallics 1990, 9, 799. (b) Masuda, K.; Saitoh, M.; Aoki, K.; Itoh, K. J. Organomet. Chem. 1994, 473, 285. (10) Rieke, R. D.; Sell, M. S.; Klein, W. R.; Chen, T.; Brown, J. D.; Hanson, M. V. In Active Metals; Fu¨rstner, A., Ed.; VCH: New York, 1996; Chapter 1. (11) Freshly acid-washed zinc powder also reduces (C5Me5)Ru(η3allyl)X2, but the reaction proceeds much more slowly. (12) Complete experimental and characterization data are provided as Supporting Information.
10.1021/om000275n CCC: $19.00 © 2000 American Chemical Society Publication on Web 06/13/2000
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prising. Such four-electron repulsive interactions have been invoked to rationalize both the structure and high reactivity of other two-electron alkyne complexes.16 However, the significantly increased kinetic lability of the electron-rich 2-butyne ligand in complex 3b relative to the π-acidic diphenylacetylene ligand of 3a is consistent with this rationalization. Although (C5Me5)Ru(η3-allyl)(η2-PhCtCPh) (3a) is stable at room temperature, allyl/alkyne coupling is induced upon heating, resulting in the near-quantitative formation of the acyclic η5-pentadienyl complex 5a (eq 3).17 The structure of air-stable 5a is established on the
3a
solid state when maintained below -20 °C, even under air. Reduction of (C5Me5)Ru(η3-allyl)X2 in the presence of either 1-(trimethylsilyl)propyne or bis(trimethylsilyl)acetylene (BTMSA) also leads to the corresponding isolable alkyne complex 3c or 3d, respectively. In addition, the η3-crotyl alkyne derivatives 4a and 4b were prepared analogously by reduction of (C5Me5)Ru(η3-crotyl)Br2 (2).9 The spectroscopic data12,13 for complexes 3 and 4 are consistent with the η3-allyl η2-alkyne formulation, although the two-electron-donor alkyne ligands exhibit 13C NMR chemical shifts minimally perturbed from those of the free alkynes.14 Rather than an indication of weak coordination, we attribute the small chemical shift differences to a balance of anisotropic shielding by the metal and the deshielding resulting from the reduction of the alkyne π-bond order, which increases its olefinic character. The metal-alkyne interaction can be further assessed with the aid of the X-ray crystal structure of complex 3a, the first structurally characterized η3-allyl η2-alkyne complex.15 The alkyne bond length is 1.261(3) Å, midway between the typical values for C-C double and triple bonds. The deviation from linearity observed for the substituents on the alkyne ligand, 29.3(2) and 29.2(2)°, confirms the significant back-bonding interaction from the metal to the alkyne. Isolable complexes incorporating an allyl and one or more alkyne ligands are very rare.3a,8a The bonding of the alkyne to the (C5Me5)Ru(η3-C3H5) fragment results in the close proximity of the orthogonal alkyne π-bonding orbital to a filled metal-based orbital, making the relative stability of these compounds particularly sur(13) Selected NMR spectroscopic data (C6D6) for 3a: 1H NMR (300 MHz) δ 8.01 (dt, J ) 8.2, 1.5 Hz, 4H, HPh), 7.29 (t, J ) 8.4 Hz, 4H, HPh), 7.10 (tt, J ) 6.9, 1.5 Hz, 2H, HPh), 3.12 (tt, J ) 9.5, 6.4 Hz, 1H, Hcentral), 2.85 (d, J ) 6.4 Hz, 2H, Hsyn), 1.47 (s, 15 H, C5Me5), 0.44 (d, J ) 9.5 Hz, 2H, Hanti); 13C{1H} NMR (75 MHz) δ 132.5, 132.3, 128.3, 126.3, 98.1, 94.1, 79.6, 44.8, 9.7. (14) Discussions of 13C NMR chemical shifts for coordinated alkynes: Templeton, J. L. Adv. Organomet. Chem. 1989, 29, 1. Kowalczyk, J. J.; Arif, A. M.; Gladysz, J. A. Organometallics 1991, 10, 1079. Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 3288. See also: Cooke, J.; Takats, J. J. Am. Chem. Soc. 1997, 119, 11088 and references therein. Werner, H.; Baum, M.; Schneider, D.; Windmu¨ller, B. Organometallics 1994, 13, 1089. (15) Crystal data for complex 3a (C27H30Ru, -80 °C): monoclinic, P21/c, a ) 7.3502(3) Å, b ) 18.5243(9) Å, c ) 16.0368(8) Å, β ) 94.2041(10)°, V ) 2177.65(18) Å3, Z ) 4, Fcalcd ) 1.390 g cm-3, µ ) 0.729 mm-1, R1 ) 0.0260, wR2 ) 0.0699 (Fo2 > -3σ(Fo2).12
basis of spectroscopic analysis12 and by comparison to previously reported (C5Me5)Ru(η5-pentadienyl) complexes prepared from intact pentadienyl precursors.18 Spectroscopic analysis reveals that the alkyne-derived phenyl groups are oriented cis in the final product. The formation of 1,2-disubstituted η5-pentadienyl ligands via the coupling of alkyne and allyl ligands has also been reported for rhenium3d and chromium3e templates. The reaction presumably proceeds via migratory coupling to give the unsaturated σ,π-vinyl olefin intermediate I (Scheme 1),19 as demonstrated in related systems.3a,e,i Scheme 1
We propose that the unsaturated ruthenium center then activates an allylic hydrogen atom, generating the Ru(IV) hydride intermediate II, which subsequently undergoes reductive elimination to yield the η5-pentadienyl product. With the exception of BTMSA complex 3d,20 the formation of 1,2-disubstituted η5-pentadienyl complexes 5 from η3-allyl alkyne complexes 3 is general for alkyland aryl-substituted alkynes. However, the temperature (16) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J. Am. Chem. Soc. 1979, 101, 585. Silvestre, J.; Hoffmann, R. Helv. Chim. Acta 1985, 68, 1461. Bruce, M. I. Chem. Rev. 1991, 91, 7. Lomprey, J. L.; Selegue, J. P. J. Am. Chem. Soc. 1992, 114, 5518. (17) For most disubstituted alkynes, the presence of excess alkyne has no significant effect on the course of the reaction. However, the reactions of acetylene, dimethyl acetylenedicarboxylate, and ethyl 2-butynoate under these conditions proceed via alternative reactivity patterns. These reactions will be discussed in a separate account: Older, C. M.; Stryker, J. M. Manuscript in preparation. (18) (a) Trakarnpruk, W.; Arif, A. M.; Ernst, R. D. Organometallics 1994, 13, 2423. (b) Trakarnpruk, W.; Arif, A. M.; Ernst, R. D. Organometallics 1992, 11, 1686. (c) Bosch, H. W.; Hund, H.-U.; Nietlispach, D.; Salzer, A. Organometallics 1992, 11, 2087. (19) For both complexes 3a and 3b, the vinyl olefin intermediate can be trapped upon addition of carbon monoxide. (20) Thermolysis of (C5Me5)Ru(η3-allyl)(η2-TMSCtCTMS) (3d) results in the formation of several unidentified products.
Communications
required to induce allyl/alkyne coupling is dependent on the alkyne: while the diphenylacetylene complex 3a requires temperatures above 50 °C to effect coupling, the 2-butyne derivative 3b undergoes insertion slowly at room temperature.12 Thermolysis of the unsymmetric alkyne complex 3c at 55 °C results in the regioselective formation of a single η5-pentadienyl product, 5c, with the large trimethylsilyl substituent located at the terminus of the pentadienyl ligand. This implies that the initial migratory allyl/alkyne coupling occurs at the sterically less hindered end of the alkyne ligand. The coupling of some alkynes is particularly facile: addition of either 1-phenylpropyne or 4-phenyl-3-butyn-2-one to complex 1 and zinc yields the η5-pentadienyl complexes 5e and 5f directly (eq 4).21
The presence of an alkyl substituent on the η3-allyl ligand complicates the allyl/alkyne reactivity manifold. Thermolysis of (C5Me5)Ru(η3-crotyl)(η2-PhCtCPh) (4a) in benzene results in the formation of an inseparable 2.2:1 mixture of two products, isolated as a yellow powder in 84% yield (eq 5). The structure of the minor
Organometallics, Vol. 19, No. 14, 2000 2663
ally assigned as the agostic η3-cyclopentenyl complex 6, on the basis of spectroscopic similarity to several closely related, well-characterized complexes of this structural class.22,23 These data also indicate that the methyl substituent on the cyclopentenyl ring is located endo to the ruthenium center, suggesting that the two regioisomeric products may arise from partitioning at the coupling stage, producing a mixture of vinyl olefin intermediates III and IV (eq 5). Only intermediate IV can undergo facile allylic activation; intermediate III instead undergoes competitive migratory cyclization, as observed in the more electrophilic systems.4-7 By comparison, attempts to induce coupling in (C5Me5)Ru(η3crotyl)(η2-MeCtCMe) (4b) lead only to decomposition. Thus, mild thermolysis of neutral ruthenium allyl/ alkyne complexes provides acyclic pentadienyl products, in sharp contrast to the cycloaddition products obtained from isostructural, but cationic, late transition metal complexes. This difference in reactivity is presumably due to the more electron-rich (C5Me5)Ru template inhibiting the rate of migratory cyclization and increasing the accessibility of the Ru(IV) oxidation state. These effects combine to promote the allylic activation pathway responsible for the formation of η5-pentadienyl complexes. By modulating the electron density of the metal template, it is thus possible to control the partitioning between cyclization and C-H bond activation pathways. Acknowledgment. Financial support from the NSERC of Canada and the University of Alberta is gratefully acknowledged. C.M.O. acknowledges an NSERC Post-graduate Scholarship and a University of Alberta Dissertation Year Fellowship. We thank Dr. Robert McDonald for the X-ray structure determination. Supporting Information Available: Text giving experimental procedures and complete spectroscopic and analytical data for all new compounds and text and tables giving details of the crystal structure determination for complex 3a. This material is available free of charge via the Internet at http://pubs.acs.org. OM000275N
product, acyclic η5-pentadienyl complex 7, is assigned on the basis of spectroscopic comparison to the diphenylacetylene adduct 5a.12 The major product is provision-
(21) The use of cyclooctyne and 2,8-decadiyne also leads to products tentatively formulated as η5-pentadienyl complexes. (22) (a) Nicholls, J. C.; Spencer, J. L. Organometallics 1994, 13, 1781. (b) Older, C. M.; Stryker, J. M. Manuscript in preparation. (23) 1H NMR spectroscopic data for complex 6 (360 MHz, C6D6): δ 7.27-7.67 (m, 10H, HPh), 4.67 (d, J ) 3.5 Hz, 1H, H2), 3.74 (dq, J ) 11.8, 5.9 Hz, 1H, H4exo), 2.94 (d, J ) 3.7 Hz, 1H, H3), 1.59 (s, 15H, C5Me5), 1.31 (d, J ) 5.9 Hz, 3H, CH3), -4.89 (d, J ) 11.9 Hz, 1H, Hagostic).
